Method and apparatus for drying solid fuels

ABSTRACT

A method for drying solid fuels prior to injection and burning in solid fuel boilers is described. High moisture content solid fuels such as bark, sludge, wet coal, etc. must be dried before they can burn. Embodiments of the present invention incorporates a fuel delivery chute integrated with the boiler wall using a flow of combustion gasses and radiant heating to partially dry the solid fuel before it reaches the combustion zone of the boiler. The combustion gasses are drawn through the delivery chute by means of the flowing fuel and air jet inductors that then blows the combustion gasses back into the boiler. A refractory wall separates the fuel chute from the boiler proper, is heated by the combustion in the boiler, and radiates heat to the fuel falling through the fuel chute.

The present application is a continuation of International PatentApplication No. PCT/US12/50610 filed Aug. 13, 2012, which claimspriority from U.S. Provisional Application 61/522,939, filed Aug. 12,2011, which are hereby incorporated by reference.

TECHNICAL FIELD OF THE INVENTION

The present invention relates to solid fuel boilers, and in particular,to a fuel chute that removed moisture from the solid fuel before thefuel enters the boiler.

BACKGROUND OF THE INVENTION

Solid fuel boilers are commonly used by industry and utilities togenerate steam for process requirements and to generate electricity.These boilers burn bark, coal, sludge, wood waste, refuse, tires, andother organic materials, often in combinations, and with fossil fuels.Generally the organic materials have high moisture contents and arestored outdoors where they are often wet from rain water or, in the caseof sludge, reclaimed from wastewater treatment facilities. In some casessome of the moisture is removed before the fuel is delivered to theboiler by means of mechanical presses or drying chambers using hotgasses from the discharge of the boiler. These practices are oftentroublesome, expensive, risky, and not particularly efficient. For thesereasons these methods are not universally employed. Even with thesemethods the fuels still contain significant moisture content. In anycase all of the water in the fuel must be removed during the combustionprocess and when the fuels are wet the combustion process can beunstable and inefficient. The situation is further exacerbated bychanging moisture contents of the fuel (e.g., from rain) that introducesvariations into the combustion process and make it more difficult tooperate the boilers.

Solid fuel boilers in this application are typically constructed aslarge boxes (up to 100 m2 or more floor area) and constructed of heavysteel tubing forming walls of the box, typically referred to as thefront, sides, and rear walls. The tubing is typically 2½″ (63.5 mm) or3″ (76.2 mm) outside diameter, arrayed in parallel relationship formingflat panels with the tubes running vertically. The tubes are typicallyspaced apart ½″ to 1″ (12 mm-25 mm) with a steel membrane bridging thegap. The whole assembly is seal welded together forming an air tightstructure. The boiler walls, or tube panels, run vertically to the topof the boiler, up to 30 m or more tall. The walls are fed re-circulatingwater by headers at their lower extremity. Typically the front walltubes are bent over more or less horizontally to form the roof of theboiler and the side walls end in relieving headers feeding back to asteam drum. The rear wall either ends in a header or feeds directly intothe steam drum. In order to feed fuel and combustion air into theboiler, and for other purposes, the boiler tubes are bent apart to formopenings in the tube panel.

The bottom of the boiler may be arranged as a grate type boiler,fluidized bed boiler, or other arrangement. Grate type boilers includetraveling grates, vibrating grates, tilting grates, and hydro-grates.Typically the grates cover the bottom of the boiler and are made ofheavy cast iron components with slots for combustion air to rise throughthe grate from a plenum below. The solid fuel lands on the grate andburns there. The ash is dumped off of the grate as the grate moves(rotates like a tank tread), vibrates, or tilts (in sections). Fluidizedbed boilers generally have a mass of sand or other media through which astream of air or boiler flue gas is percolated to fluidize the bed. Thefluidized bed acts as a heat sink, fuel drying system, turbulentfuel/air mixing system, fuel distribution system, and means forseparating fuel and ash in the boiler. Additional combustion air ports,typically called “over fired air” (OFA) are arranged to blow air inabove the grate or fluidized bed to help complete the combustion. In allof these arrangements if the fuel is not properly dried poor combustioncan result leading to poor operational efficiencies and highenvironmental emissions.

In common practice the solid fuel is fed by gravity through largechutes, steeply mounted and about two feet (600 mm) or more square, froma hopper and/or conveyor system above, to the lower portion of theboiler just above the grate or fluidized bed. There are typicallymultiple chutes penetrating a wall or walls of the boiler. A solid fueldistributor is often integral with and at the bottom of the chute rightat the interface with the boiler wall. Mechanical distributors andpneumatic distributors are commonly used. Grate type boilers generallyrequire some type of fuel distribution whereas fluidized bed boilers canbe run without them as the fluidized bed can distribute the fuel, albeitinefficiently. Typically the fuel slides down the chute and enters theboiler with high residual moisture content (up to 50% or more). Thewater in the fuel inhibits the combustion in the furnace often requiringthe continual use of supplemental fossil fuels to provide additionalheat to compensate for the moisture. It is also very common for the loadrate on these boilers to change frequently in reaction to changing steamdemands. Inconsistent and high moisture content of the fuels makes itdifficult for the boiler to respond effectively to the required loadchanges. This requires, again, the use of supplemental fossil fuels toimprove the response of the boiler to load rate changes. Fossil fuelsare typically used to start these boilers but continual use of fossilfuels is extremely expensive. Fluidized beds can help to compensate forvarying moisture contents and load rates because they act as heat sinks,but they are limited in their heat release rate and can have significantoperational and mechanical problems such as sand sintering and sanderosion and they require a sand reclamation system.

There is great demand for a simple means to dry solid fuels so that theyare delivered to the boiler combustion chamber ready to burn. Such asystem must be inexpensive to install and operate, reliable, effective,and safe. The present invention, embodiments of which are describedbelow, addresses this challenge and incorporates all of these features.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional side elevation of the lower part of the combustionchamber of a water tube boiler with embodying the present invention.

FIG. 2 is a partial sectional side view elevation of the boiler ofFIG. 1. To the right of the drawing is the interior of the boiler wherethe fuel is burned. To the left of the drawing is outside the boiler.

FIG. 3 is a sectional plan view through the embodiment of FIGS. 1 and 2,identifying the various components of the boiler.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

Referring to FIG. 1, the lower part of the combustion chamber 1 of awater tube boiler is constructed of heavy gauge vertically aligned steeltubes 2 forming tube walls 3 enclosing the combustion chamber on foursides. The bottom of the boiler may be arranged with a traveling grate,vibrating grate, fluidized bed, or stepped floor 4 as shown (referenceU.S. patent application Ser. No. 12/557,085). Bulk fuel such as bark maybe injected 5 from a fuel distributor 23 and land on the floor 4 andburn there. Combustion gases 6 are emitted from the burning fuel 7 andrise and exit the combustion chamber at 8 on their way to the convectiveheat transfer sections of the boiler (not shown).

Referring now to FIG. 2 and FIG. 3, an upper fuel chute 9 descends fromabove at a steep angle and intersects vertical chute 10 with a verticaldimension determined by the available height of the boiler or otherlimiting factors. In general vertical chute 10 is made as tall aspossible. Vertical chute 10 is arranged immediately adjacent to theboiler tube wall 3 and is made from boiler tubes 11 arranged to form athree sided chute with two parallel sides 12 perpendicular to boilertube wall 3 and a back side 13 parallel to but offset from the boilertube wall 3 that bridges between the parallel sides 12. The boiler tubes14 in the plane of tube wall 3 that are between the two parallel sides12 are bent out of the plane of the tube wall 3 to feed water to tubescomprising the walls 12 and 13 of the vertical chute 10. When thesetubes are bent out of the plane of tube wall 3 an opening 15 is createdin tube wall 3 exposing the interior of the vertical chute 10 to thecombustion chamber. Vertical chute 10 terminates at its bottom in asloping floor 16 descending back to the plane of tube wall 3. The sides12 and back 13 of vertical chute 10 are lined with one or more layers ofinsulating material 17 and/or refractory 18. Insulating material 17 andrefractory material 18 may be cast in place or prefabricated as boardsor tiles. High temperature resistant refractory 19 is arranged to bridgegap 15 and close off most of vertical chute 10 from combustion chamber1. Refractory 19 may be cast in place or prefabricated as blocks ortiles. Refractory 19 is arranged to leave openings 20 and 21 at the topand bottom respectively between the combustion chamber 1 and verticalchute 10. The floor 16 of vertical chute 10 is lined on the upper sidewith an abrasion resistant refractory or ceramic material 22. A fueldistributor 23 can be arranged to fit at the bottom of vertical chute10. Nozzles 26 and 27 may be incorporated to inject recirculated fluegas as described below.

Wet fuel enters chute 9 at point 24 and flows into vertical chute 10 atpoint 25. The fuel then falls through vertical chute 10 until it landson sloping floor 16 and slides down to the fuel distributor 23 and isthen injected through opening 21 into the boiler at 5. The motion of thefalling fuel will induce a downward flow of combustion gases throughvertical chute 10. The gases will enter the chute at the top throughopening 20 and exit the chute at the bottom through opening 21. The hotcombustion gas (around 2000 F) will partially dry the fuel as it isfalling through vertical chute 10. Refractory 19 will also be heated bythe combustion in combustion chamber 1 and reach an equilibriumtemperature around 2000 F. Refractory 19 will then radiate to theinterior of vertical chute 10 increasing the heat transfer to thefalling fuel and further drying the fuel. The liberated water vapor willexit vertical chute 10 through the lower opening 21 with the flowingboiler gas and fuel. Nozzles 26 are arranged to inject recirculated fluegas into the top of chute 9 to purge high temperature gases that mayaccumulate there and to help induce the flow of combustion gases throughvertical chute 10. Nozzles 27 are located to help blow the fuel down tofuel distributor 5 and to help induce the flow of combustion gas throughvertical chute 10. These nozzle or other nozzles may be installed inother advantageous locations. The recirculated flue gas is tapped off ofthe flue gas ducts preferably downstream of all heat transfer surfaces,pollution control equipment, and fans. It is then routed through abooster fan and injected at a higher pressure through nozzles 26 and 27.

Embodiments of the present invention share some characteristics with thesecond embodiment described in patent application Ser. No. 12/471,081,filed May 22, 2009 by some of the same inventors. In that applicationthe second embodiment describes a fuel chute for drying fuel in which anupper chute controls the flow of fuel to a lower chute, the lower chutehaving three sides made from boiler tubes and the fourth side open tothe interior of the boiler. In that embodiment the fuel falling throughthe lower chute is exposed to the combustion in the boiler and heated bydirect radiation and whatever combustion gasses the fuel contacts. Insome embodiments of the present invention, the three sided chute isseparated from the interior of the boiler by the refractory walldescribed above. The refractory wall is open at the top and bottom andcombustion gasses are induced to flow from the boiler into the chute atthe top and back into the boiler at the bottom, thereby promoting themixing of the hot gasses with the fuel. The refractory wall is in placeto contain the falling fuel inside the chute so that it will flowproperly to the fuel distributor. The refractory wall has the secondfunction of absorbing heat from the combustion inside the boiler andthen radiating heat to the fuel inside the chute. Embodiments of thepresent invention also incorporates refractory lining of the lower chutefor insulation and abrasion resistance; gas-jets to help induce the flowof gases through the chute and promote the movement of the fuel andcooling of the upper chute; a refractory or ceramic lined sloping floor;and an integral fuel distributor.

It will be apparent to those skilled in the art that many changes andmodifications may be made to the system described above withoutdeparting from the invention in its broader aspects. The appended claimsare therefore intended to cover all such changes and modifications asfall within the true spirit and scope of the invention.

We claim as follows:
 1. An apparatus for drying fuel entering a solidfuel boiler having a combustion chamber within tube walls carrying fluidto be heated by combustion in the combustion chamber, the apparatuscomprising: a fuel chute for transferring solid fuel to the combustionchamber, the chute having an interior region through which fuels falls,the interior region being separated from the interior of the combustionchamber by a refractory material that transfers heat from the combustionchamber to the solid fuel as it drops in the chute to reduce themoisture content of the solid fuel before it reaches the combustionchamber; an entrance for inserting solid fuel into the fuel chute; andan exit in the fuel chute for dispensing solid fuel into the combustionchamber; the chute wall transferring heat to the solid fuel as it dropsin the chute to reduce the moisture content of the solid fuel before itreaches the combustion chamber.
 2. The apparatus of claim 1 in which therefractory material is heated by the combustion chamber and heats thefuel by radiation.
 3. The apparatus of claim 1 in which the refractorymaterial comprises of tiles, board, or other prefabricated components.4. The apparatus of claim 1 in which the refractory material is a castin place material.
 5. The apparatus of claim 1 in which the refractorymaterial reaches an equilibrium temperature of at least 2000° F.
 6. Theapparatus of claim 1 in which solid fuel includes bark, coal, sludge,wood, waste, refuse, tires, or fossil fuels having a residual moisturecontent of 50% or greater prior to entering the chute.
 7. The apparatusof claim 1 in which the chute is positioned between a tube wall and thecombustion chamber.
 8. The apparatus of claim 1 in which the interior ofthe chute is lined on four sides with refractory material.
 9. Theapparatus of claim 1, further comprising a fuel distributor thatdispenses fuel to the combustion chamber from the fuel chute.
 10. Theapparatus of claim 1 in which the fuel chute includes an opening for gasfrom the combustion chamber to enter the fuel chute, the gas being drawninto the chute by the flow of fuel down the chute and mixing with thefuel as it descends the chute.
 11. The apparatus of claim 1, furthercomprising a gas nozzle for injecting gas into the portion top of thechute.
 12. A solid fuel boiler including: tube walls for carrying fluidto be heated; a combustion chamber for combusting fuel to heat the fluidin the rube walls; and an apparatus for drying fuel in accordance withclaim
 1. 13. A method for providing solid fuel to a combustion chamberof a solid fuel boiler, comprising: inserting the fuel into a gravityfed chute through an entrance in the gravity fed chute; reducing themoisture content of the solid fuel while the fuel is passing through thegravity fed chute by heating the fuel in the chute by radiation from atleast one wall of the gravity fed chute that is heated by thermalcontact with the combustion chamber; and delivering the fuel with thereduced moisture content into the combustion chamber.
 14. The method ofclaim 13 further comprising drawing gas from the combustion chamber intothe chute near the top of the chute, the gas flowing down the chute withthe fuel.
 15. The method of claim 13 further comprising injecting gasinto the chute near the top to circulate gas at the top of the chute.16. The method of claim 13 in which reducing the moisture content of thesolid fuel while the fuel is passing through the gravity fed chute byheating the fuel in the chute by radiation from at least one wallcomprises passing the fuel through a chute that is separated from theinterior of the combustion chamber by a refractory material and not by atube wall.
 17. The method of claim 13 in which delivering the fuelincludes the fuel dropping onto a sloping floor of the gravity fedchute, the sloping floor lined with abrasion resistant refractorymaterial.
 18. The method of claim 13 in which delivering the fuelincludes injecting gas into the gravity fed chute to help deliver thesolid fuel to the combustion chamber.
 19. An apparatus for drying fuelentering a solid fuel boiler having a combustion chamber within tubewalls carrying fluid to be heated by combustion in the combustionchamber, the apparatus comprising: a fuel chute for transferring solidfuel to the combustion chamber, the chute having at least one chute wallin thermal contact with the combustion chamber, the chute wall not beingseparated from the interior of the combustion chamber by a tube wall; anentrance for inserting solid fuel into the fuel chute; and an exit inthe fuel chute for dispensing solid fuel into the combustion chamber,the at least one chute wall transferring heat to the solid fuel as itdrops in the chute to reduce the moisture content of the solid fuelbefore it reaches the combustion chamber.
 20. The apparatus of claim 19in which the at least one chute wall in thermal contact with thecombustion chamber comprises a refractory material.